Implicit variable binding with type instance and explicit forall

Summary

Variables in signatures on the right hand side of a type instance are imperoply bound when there is an explicit forall.

Steps to reproduce

{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DataKinds #-}

module ScopeProblem where

type family A :: k
type instance forall. A = Nothing :: Maybe a -- 'a' should be out of scope

class Foo x where
  type B x :: Maybe a
  type forall x. B x = Nothing :: Maybe a -- 'a' should be out of scope

instance Foo [x] where
  type forall. B [x] = Nothing :: Maybe a -- 'a' should be out of scope

Expected behavior

a should be out of scope in all 3 cases.

Environment

• GHC version used: 8.10.1